This invention relates to the measurement of electric current in a non-contact manner utilizing current transformers. More particularly, the invention relates to the use of current transformers to measure a-c (alternating-current) electric current in the case of a d-c (direct-current) electric current component also being present. One important application is in electric energy metering, providing a way to improve current transformer accuracy in the presence of a d-c current component, thereby improving the accuracy of the energy meter.
Most current monitoring systems for a-c electric power systems utilize ordinary current transformers to provide input currents that are isolated from the electric power system conductors, similar to FIG. 1. A current-carrying conductor 4 is configured as a primary winding of a current transformer CT1, and is magnetically coupled to a magnetic core 1. For clarification, the term “magnetic core” as used herein refers to a magnetic body having a defined relationship with one or more conductive windings. A secondary winding 2 is also magnetically coupled to magnetic core 1. The phrase “magnetically coupled” is intended to mean that flux changes in a magnetic body are associated with an induced voltage in the winding, the induced voltage being proportional to the rate of change of magnetic flux that is coupled, in accordance with Faraday's Law.
A secondary electric current J2 is induced in the secondary winding and is approximately proportional to a primary electric current J1. The secondary current is isolated from the primary current and is smaller than the primary current by the turns ratio of the primary and secondary windings. The primary winding may consist of only one turn (as in FIG. 1) or may have multiple turns wrapped around the magnetic core. The secondary winding usually consists of multiple turns wrapped around the magnetic core. If the current transformer had ideal properties, the secondary current would be instantaneously and precisely proportional to the primary current. However, since magnetic cores with ideal properties are not known, the current transformer is subject to a substantial error of ratio, phase-angle, and/or wave-shape in its output as a function of                (a) properties of the magnetic core,        (b) its burden, and        (c) a d-c component of the primary current.(The “output” of a current transformer is usually considered to be its secondary current). The impact that each of these items has on current transformer accuracy will now be discussed briefly.        
The magnetic core of a current transformer plays an important part in determining current transformer accuracy. Current transformer accuracy is usually acted by the coercive force of the magnetic core material (less is better), the cross sectional area of the magnetic core (bigger is better), the effective magnetic length of the magnetic core (shorter is better), any air gap in the magnetic core (less or none is usually better), and the “squareness” of the magnetic core material hysteresis curve (squarer may be preferred if not operating near saturation, otherwise characteristics that are not square may be preferred). Split-core current transformer cores generally have hysteresis curves that are less square than standard current transformer cores due to the small air gaps inherent in the design of split-core current transformers.
Additionally, the permeability of the magnetic core material is important. Permeability is usually defined as a ratio of magnetic flux density (B) to magnetic field strength (H). In the absence of a d-c current component, current transformer accuracy normally improves as the permeability of the magnetic core material increases. For purposes of this disclosure, several types of permeability are significant. “Normal permeability” refers to the simple ratio of B/H at any specified point on a hysteresis curve. “Initial permeability” refers to the ratio of B/H at very small magnitudes of H (usually tested after the material has been demagnetized). “Incremental permeability” refers to the ratio of a change of magnetic flux density relative to a small change of magnetic field strength (AB/AH) at any specified location on a hysteresis curve.
While the permeability of magnetic core material is an important factor in current transformer accuracy, it is equally important how the magnetic core is constructed. Current transformer accuracy is dependent on the properties of the total magnetic core configuration, not just the magnetic material that the core is made with. While “permeability” is an important parameter of the magnetic material, “permeance” is an important parameter of the magnetic core. Permeance is usually defined as a ratio of magnetic flux (Φ) (through a cross-section of a magnetic circuit) to magnetomotive force (F). Permeance is the reciprocal of reluctance. In the absence of a d-c current component, current transformer accuracy normally improves as the permeance of the magnetic core increases. For purposes of this disclosure, several types of permeance are significant. “Normal permeance” refers to the simple ratio of Φ/F at any specified point on a hysteresis curve. “Initial permeance” refers to the ratio of Φ/F at very small magnitudes of F (usually tested after the magnetic core has been demagnetized). “Incremental permeance” refers to the ratio of a change of magnetic flux relative to a small change of magnetomotive force (ΔΦ/ΔF) at any specified location on a hysteresis curve.
Current transformer accuracy is also affected by the burden of the current transformer secondary circuit. In order for the secondary current generated by a current transformer to be an accurate representation of the primary current, the impedance of the circuit connected to the secondary winding must be kept low so that current can flow freely. The impedance of the secondary circuit is often called the “burden.” The burden generally includes all impedances in the loop through which the secondary current flows, including stray winding impedances, stray impedances of connecting conductors, and the impedances of any devices connected in the loop (such as current-sensing resistors and relay operating coils). In order for a current transformer to drive a secondary current through a non-zero burden, a voltage must be induced in the secondary winding. The induced voltage is proportional to secondary current and is proportional to the burden, in accordance with Ohm's Law (the induced voltage equals the secondary current times the vector sum of all secondary loop impedances). The induced voltage is induced in the secondary winding by a fluctuating magnetic flux in the magnetic core, the instantaneous magnitude of induced voltage being proportional to the rate of change of magnetic flux, in accordance with Faraday's Law. The fluctuating magnetic flux is associated with an “exciting current” in accordance with well-known electromagnetic principles. The exciting current is often understood to have a magnetizing component and a core loss component. When utilized to measure alternating current with no d-c component, the exciting current accounts for the error in the secondary current, and may be referred to herein as an “exciting current error.” Generally speaking, the accuracy of a current transformer is inversely related to the burden of the secondary circuit. A higher burden causes the current transformer to operate with greater induced voltage, thereby increasing the exciting current error, thereby causing the secondary current to be less accurately proportional to the primary current.
Current transformer accuracy is also affected by a d-c current component that may be present in the primary current. A d-c current component causes the magnetic core to experience a large magnetomotive force, which can cause the core to saturate, and thereby cause severe distortion of the secondary current. For clarity, magnetomotive force will now be discussed briefly.
Magnetomotive force (F) is associated with the art of magnetic circuits, and is often defined for a closed loop as the line integral of the magnetic field strength (H) around the closed loop:F=H·dl 
Magnetomotive force F is a scalar quantity associated with the closed loop, while magnetic field strength H is a vector quantity. By Ampere's Law, magnetomotive force is proportional to the total current flowing through the closed loop. Utilizing the meter-kilogram-second (m.k.s.) system of units, magnetomotive force has units of amperes (or amp-turns), and is equal to the total current flowing through the closed loop. The closed loop is often chosen to pass through one or more conductive windings wrapped around a magnetic body, and a magnetomotive force equal to the current in the winding times the number of winding turns is associated with each winding (thus the unit “amp-turns”). The total instantaneous magnitude of magnetomotive force for the closed loop is the instantaneous sum of magnetomotive force contributions from all windings and other conductors that pass through the closed loop.
Many magnetic devices operate best with an average magnetomotive force near zero. A deviation away from zero often results in excessive buildup of magnetic flux that causes the device to malfunction. Ordinary current transformers are one type of device for which this is usually the case.
With preferred current transformer operation, the amp-turns of the primary winding are largely canceled by the amp-turns of the secondary winding, so that the magnetomotive force acting on the current transformer core is relatively small. The net magnetomotive force acting on the core is equal to the sum (or difference, depending on polarity conventions) in amp-turns of the primary winding and the secondary winding, and this sum is proportional to a secondary current error.
Speaking more precisely of current transformer operation, a secondary electric current error is proportional to the magnetomotive force acting on the magnetic core. The instantaneous value of the magnetomotive force is equal to the instantaneous sum of the primary electric current multiplied by the number of turns of the primary winding and the secondary electric current multiplied by the number of turns of the secondary winding (with the primary current and secondary current having opposite polarities so that their magnetomotive force contributions tend to cancel each other). The secondary electric current error comprises a d-c component and an a-c component; the d-c component will be referred to as a d-c current error, and the a-c component will be referred to as an exciting current error.
Ordinary current transformers work properly only with alternating primary current. When a d-c component is present in the primary current, normal current transformer operation cannot maintain a d-c component in the secondary circuit, and a large d-c current error results. This d-c current error correlates to a large d-c magnetomotive force applied to the magnetic core, which causes the magnetic core to saturate, thereby adversely affecting current transformer operation.
A great many variations to the basic current transformer circuit have been developed in the prior art to improve current transformer accuracy for various applications. Some of these are summarized here:                (a) Utilize an active load to sense current. An active load can have an effective burden of virtually zero Ohms, but this does not solve the problem of stray impedances contributing to the burden of the secondary circuit. The use of an active load to reduce current transformer burden is described in detail in U.S. Pat. No. Re. 28,851 to Milkovic (reissued 1976) for a “Current Transformer with Active Load Termination.”        (b) FIG. 2 illustrates a prior-art “zero-flux” current transformer. This is one form of a current transformer having an electronic assist means. A sense winding 10 terminated in a high-impedance manner provides a voltage signal V4 that is proportional to the rate of change of magnetic flux. By amplifying this signal and applying it in series with the secondary winding, the effective burden of the entire secondary circuit is reduced to near zero ohms. Magnetic flux changes in the current transformer core are reduced to near zero, and the exciting current required is reduced to near zero, thereby making secondary current more accurately proportional to primary current. The amplifier essentially provides the driving voltage necessary to drive loop current through secondary loop impedances so that the current transformer core does not need to generate this voltage via a changing flux. Higher gains in the amplifier circuit contribute to increased accuracy and smaller flux changes, though excessively high gain typically leads to instability and associated oscillations. This device provides very good accuracy for measurement of a-c current, but measurement accuracy is significantly reduced by the presence of a d-c current component in the primary current.        (c) In order to measure currents with d-c components, Hall-effect current sensors are often used. These sensors typically insert a Hall-effect magnetic field sensor in a current transformer core air gap. In “open loop” devices, the magnetic field strength is used to estimate the primary current directly. “Closed loop” devices utilize a zero-flux concept similar to that described for FIG. 2. However, instead of using a sense winding (as in FIG. 2), the Hall-effect element generates a voltage signal proportional to the magnetic field in the air gap. A high-gain amplifier circuit is used to drive secondary current to continuously nullify the magnetic field, which causes the secondary winding amp-turns to balance the primary winding amp-turns. This results in a secondary current that is proportional to the primary current. A current-sensing resistor in the secondary circuit normally provides a voltage signal that is proportional to secondary current. While these Hall-effect current sensors are widely used, their accuracy and stability over time are not adequate for many applications.        (d) FIG. 3 shows another prior-art current transformer circuit that operates with an electronic assist means. This type of “burden-reducing” circuit is described in U.S. Pat. No. 6,522,517 by Edel. This patent in its entirety is hereby incorporated by reference into this disclosure.        The circuit shown in FIG. 3 uses the secondary current as an input to generate the compensation voltage required to drive secondary current (voltage V1 is proportional to secondary current J2, and is used as an information signal to produce output voltage V3). This circuit has the advantage of utilizing ordinary current transformers without the need for a sense winding or Hall-effect sensor. However, the circuit shown in FIG. 3 can only be used to measure a-c current. The associated patent describes how the control circuit can be modified to enable accurate measurement of current with a d-c current component, but the method used is dependent on brief periodic reset pulses applied to the magnetic core, during which time current cannot be measured.        (e) Many specialized current transformers with multiple windings and multiple cores have been developed. Many of these transformers have excellent accuracy. However, most of these specialized transformers are prohibitively expensive for many applications. Some devices having simple magnetic cores drive the core in and out of saturation to measure d-c current, often causing excessive noise on the primary circuit.        
It is therefore an object of the present invention to provide an economical current sensor with the following properties:                (a) Provide an a-c current transformer that has good accuracy even with the presence of a d-c component in the primary current.        (b) Utilize a relatively simple and inexpensive magnetic core.        (c) Have a high degree of stability over time and temperature.        (d) Not cause noise on the primary circuit.        
Other objects and advantages will become apparent from the description of the invention.